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Nigeria's army, with the help of regional troops, has now recaptured most territory from Boko Haram

Nigerian ground troops have joined an offensive on the last known hideout of the Boko Haram Islamist militants, a military spokesman has told the BBC.

The vast north-eastern Sambisa forest is where they have many bases - and it has been subject to aerial bombardments since February.

There has been speculation that some of the Chibok schoolgirls kidnapped more than a year ago are being held there.

Boko Haram has killed thousands in northern Nigeria since 2009.

Nigeria's military, backed by troops from neighbouring countries, launched an offensive against Boko Haram in February - and has recaptured most of the territory the militants had taken in the previous year.